Okay, I did a little more digging and it seems you're right eszed! Even undersea cables can increase their bandwidth, at least in some cases; seems to be dependent on some modulation factors that I don't fully understand.

If I'm reading this right, BPSK - used on the longest transpacific cables - can't really be upgraded to have higher bandwidth but QPSK and other modulation schemes used for shorter distances can be (?).